Color organic pigments and electrophoretic display media containing the same

ABSTRACT

An electrophoretic display medium includes a front and a rear electrode, at least one of the front and rear electrodes being transparent, and an encapsulated dispersion fluid containing a plurality of pigments positioned between the front and rear electrode. The plurality of pigments includes a first and a second type of organic pigment particle. The first type of organic pigment particle has a first color and a first charge polarity. The second type of organic pigment particle has a second color different from the first color and a second charge polarity the same as the first charge polarity. At least one of the first and second types of organic pigment particle includes a silica coating and a polymeric stabilizer covalently bonded to the silica coating.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application having Ser. No.15/869,578 filed on Jan. 12, 2018, which claims priority to and thebenefit of U.S. Provisional application having Ser. No. 62/448,683 filedon Jan. 20, 2017, the content of which is incorporated by referenceherein in their entireties.

FIELD OF THE INVENTION

This invention relates to organic pigments used in electrophoreticdisplay media. More specifically, in one aspect this invention relatesto electrophoretic systems containing multiple differently coloredorganic pigments having similar charge polarity.

BACKGROUND OF INVENTION

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence, or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, such as black and white, and such that after anygiven element has been driven, by means of an addressing pulse of finiteduration, to assume either its first or second display state, after theaddressing pulse has terminated, that state will persist for at leastseveral times, for example at least four times, the minimum duration ofthe addressing pulse required to change the state of the displayelement. Some particle-based electrophoretic displays are stable notonly in their extreme black and white states but also in three or morestates, such as multi-color displays having three or more colors. Forconvenience the term “bistable” may be used herein to cover displayelements having two or more display states.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC and related companies describe various technologies usedin encapsulated and microcell electrophoretic and other electro-opticmedia. Encapsulated electrophoretic media comprise numerous smallcapsules, each of which itself comprises an internal phase containingelectrophoretically-mobile particles in a fluid medium, and a capsulewall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. In a microcell electrophoreticdisplay, the charged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. Thetechnologies described in these patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 7,002,728 and 7,679,814;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. Nos. 6,922,276 and 7,411,719;

(c) Microcell structures, wall materials, and methods of formingmicrocells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;

(d) Methods for filling and sealing microcells; see for example U.S.Pat. Nos. 7,144,942 and 7,715,088;

(e) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178 and 7,839,564;

(f) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318 and7,535,624;

(g) Color formation and color adjustment; see for example U.S. Pat. Nos.6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714;6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502;7,167,155; 7,385,751; 7,492,505; 7,667,684; 7,684,108; 7,791,789;7,800,813; 7,821,702; 7,839,564; 7,910,175; 7,952,790; 7,956,841;7,982,941; 8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076;8,363,299; 8,422,116; 8,441,714; 8,441,716; 8,466,852; 8,503,063;8,576,470; 8,576,475; 8,593,721; 8,605,354; 8,649,084; 8,670,174;8,704,756; 8,717,664; 8,786,935; 8,797,634; 8,810,899; 8,830,559;8,873,129; 8,902,153; 8,902,491; 8,917,439; 8,964,282; 9,013,783;9,116,412; 9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646;9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511; 9,341,916;9,360,733; 9,361,836; 9,383,623; and 9,423,666; and U.S. PatentApplications Publication Nos. 2008/0043318; 2008/0048970; 2009/0225398;2010/0156780; 2011/0043543; 2012/0326957; 2013/0242378; 2013/0278995;2014/0055840; 2014/0078576; 2014/0340430; 2014/0340736; 2014/0362213;2015/0103394; 2015/0118390; 2015/0124345; 2015/0198858; 2015/0234250;2015/0268531; 2015/0301246; 2016/0011484; 2016/0026062; 2016/0048054;2016/0116816; 2016/0116818; and 2016/0140909;

(h) Methods for driving displays; see for example U.S. Pat. Nos.7,012,600 and 7,453,445;

(i) Applications of displays; see for example U.S. Pat. Nos. 7,312,784and 8,009,348; and

(j) Non-electrophoretic displays, as described in U.S. Pat. No.6,241,921 and U.S. Patent Applications Publication No. 2015/0277160; andapplications of encapsulation and microcell technology other thandisplays; see for example U.S. Patent Application Publications Nos.2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electrophoretic display, which is intendedfor use with a stylus, print head or similar movable electrode separatefrom the display, only one of the layers adjacent the electrophoreticlayer comprises an electrode, the layer on the opposed side of theelectrophoretic layer typically being a protective layer intended toprevent the movable electrode damaging the electrophoretic layer.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

However, the service life of encapsulated electrophoretic displays, isstill lower than is altogether desirable. It appears that this servicelife is limited by factors such as the tendency of particles toaggregate into clusters which prevent the particles completing themovements necessary for switching of the display between its opticalstates. The physical properties and surface characteristics ofelectrophoretic particles can be modified by adsorbing various materialsonto the surfaces of the particles, or chemically bonding variousmaterials to these surfaces. For example, in an electrophoretic displaythat contains organic pigments, monomers having different chemicalgroups may form polymer coatings on the pigments by dispersionpolymerization and the coatings may react with a charge control agent toprovide colored particles of varying charge strength. It has beenobserved, however, that as the number of colors increase, some of thedifferently colored polymer-coated pigments may have difficultyseparating from one another due to the similarity of the coated polymerstructures. An alternative approach is to use inorganic color pigment,but the color strength and brightness of organic pigments is superior toinorganic pigments. Because organic pigments are preferred, there is aneed for improved color electro-optic displays that includeelectrophoretic media containing a plurality of colored organicparticles.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an electrophoreticdisplay medium comprises a front and a rear electrode and anencapsulated dispersion fluid containing a plurality of pigmentspositioned between the front and rear electrode. At least one of thefront and rear electrodes may be transparent. The plurality of pigmentscomprise a first and a second type of organic pigment particle. Thefirst type of organic pigment particle may have a first color and afirst charge polarity. The second type of organic pigment particle mayhave a second color different than the first color and a second chargepolarity that is the same as the first charge polarity. At least one ofthe first and second types of organic pigment particle includes a silicacoating and a polymeric stabilizer bonded to the silica coating.

These and other aspects of the present invention will be apparent inview of the following description.

BRIEF DESCRIPTION OF THE FIGURES

The drawing FIGURE depicts one implementation in accord with the presentconcepts, by way of example only, not by way of limitation.

The FIGURE depicts an electrophoretic display device according to oneembodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced without suchdetails.

According to one embodiment of the present invention, an electrophoreticdisplay medium is provided that may be incorporated into anelectro-optic display. The electro-optic display may comprise a frontand a rear electrode, at least one of the front and rear electrodesbeing transparent, and an encapsulated dispersion fluid containing aplurality of pigments positioned between the front and rear electrode.The plurality of pigments may comprise a first and a second type oforganic pigment particle. The first type of organic pigment particle mayhave a first color and a first charge polarity, and the second type oforganic pigment particle may have a second color and a second chargepolarity. The first color and second color may also be different, whilethe first and second charge polarity are the same.

The electrophoretic display medium may optionally further comprise athird type of organic pigment having a third color and third chargepolarity, both the third color and third charge polarity being differentthan the first and second color and charge polarity. Each of the first,second, and third types of organic pigment particles may include, butare not limited to, CI pigment PR 254, PR122, PR149, PG36, PG58, PG7,PB28, PB15:1, PB15:2, PB15:3, PB15:4, PY83, PY138, PY150, PY 151, PY154,PY155 or PY20, as well as other commonly used organic pigments describedin color index handbooks, “New Pigment Application Technology” (CMCPublishing Co, Ltd, 1986) and “Printing Ink Technology” (CMC PublishingCo, Ltd, 1984). Specific examples include Clariant Hostaperm Red D3G70-EDS, Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70,Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm YellowHR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia RedL 4100 HD, and Irgazin Red L 3660 HD; Sun Chemical phthalocyanine blue,phthalocyanine green, diarylide yellow or diarylide AAOT yellow.Furthermore, the color of the first, second, and third types of organicpigment particles may be independently colored red, green, blue, cyan,magenta, or yellow, for example.

In addition to the colors, the particles may have other distinct opticalcharacteristics, such as optical transmission, reflectance, luminescenceor, in the case of displays intended for machine reading, pseudo-colorin the sense of a change in reflectance of electromagnetic wavelengthsoutside the visible range.

According to the various embodiments of the present invention, at leastone of the first and second types of organic pigment particles include asilica coating to which a polymeric stabilizer may be bonded. In oneembodiment, the polymeric stabilizer may comprise a polymer containing asilane coupling group and the silane group is covalently bonded to thesilica coating. In another embodiment, the polymeric stabilizer may beionically bonded to a silane coupling agent having a silane group, andthe silane group may be covalently bonded to the silica coating.

Referring now specifically to the FIGURE, the electrophoretic fluid maycomprise four types of particles dispersed in an encapsulated dispersionfluid, such as a dielectric solvent or solvent mixture. For ease ofillustration, the four types of pigment particles may be referred to asthe first type (11), the second type (12), the third type (13) and thefourth type (14) of particles, as shown in the FIGURE. However, withonly four types of pigment particles, a display device utilizing theelectrophoretic fluid may display at least five different color states,which leads to a full color display. The dispersion fluid may beencapsulated according to any method known to those of skill in the art,e.g. microcapsules, microcells, or a polymer matrix, and to any size orshape, such as spherical, for example, and may have diameters in themillimeter range or the micron range, but are preferably from about tento about a few hundred microns.

Various coating methods may be employed to provide the organic pigmentparticle with a silica coating. For example, the coating methoddescribed in U.S. Pat. No. 3,639,133, the contents of which areincorporated herein by reference in its entirety, provides an example ofa coating method. Prior to coating the organic pigment particles, theparticles may be prepared by first de-agglomerating and homogenizing anaqueous slurry of the organic pigment particles using various knownmethods, such as sonication, ball milling, jet milling, etc. Adispersant may be added to the aqueous slurry to maintainde-agglomeration of the pigment particles. In one process, the organicpigment particles are dispersed in a solution of ethanol and tetraethylorthosilicate and react at room temperature for 20 hrs under basicconditions to form a generally uniform coating of silica over theparticles. The coating is preferably 0.5 to 10 nm thick, more preferably1 to 5 nm.

After the deposition of the silica coating is complete, the pH of thereaction mixture may be reduced below about 4, and preferably to about3, before the silica-coated particles are separated from the reactionmixture. The reduction in pH is conveniently effected using sulfuricacid, although other acids, for example, nitric, hydrochloric andperchloric acids, may be used. The particles are conveniently separatedfrom the reaction mixture by centrifugation. Following this separation,it is not necessary to dry the particles. Instead, the silica-coatedparticles can be readily re-dispersed in the medium, typically anaqueous alcoholic medium, to be used for the next step of the processfor the formation of the polymeric stablizer on the particles. Thisenables the silica-coated pigment particles to be maintained in anon-agglomerated and non-fused form as they are subjected to theprocesses for attachment of polymerizable or polymerization-initiatinggroups, thus allowing for thorough coverage of the pigment particle withsuch groups, and preventing the formation of large aggregates of pigmentparticles in the microcapsules which will typically eventually be formedfrom the silica-coated pigment. Preventing the formation of suchaggregates is especially important when the silica-coated pigment is tobe used in small microcapsules (less than about 100 μm in diameter), andsuch small microcapsules are desirable since they reduce the operatingvoltage and/or switching time of the electrophoretic medium.

According to a first embodiment of the present invention, the polymericstabilizer may be derived from one or more monomers or macromonomersusing various polymerization techniques known by those of skill in theart. For example, the polymeric stabilizer on the silica coated organicpigment particles may be obtained by random graft polymerization (RGP),ionic random graft polymerization (IRGP), and atom transfer radicalpolymerization (ATRP), as described in U.S. Pat. No. 6,822,782, thecontents of which are incorporated herein by reference in its entirety.As used herein throughout the specification and the claims,“macromonomer” means a macromolecule with one end-group that enables itto act as a monomer.

Suitable monomers for forming the polymeric stabilizer may include, butare not limited to, styrene, alpha methyl styrene, methyl acrylate,methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butylacrylate, t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone,2-hydoxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethylmethacrylate, lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate,n-octadecyl methacrylate, 2-perfluorobutylethyl acrylate, 2,2,2trifluoroethyl methacrylate, 2,2,3,3 tetrafluoropropyl methacrylate,1,1,1,3,3,3-hexafluoroisopropyl acrylate,1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl acrylate, 2,2,3,3-tetrafluoropropylacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, and2,2,3,3,4,4,4-heptafluorobutyl methacrylate or the like. Themacromonomer may contain a terminal functional group selected from thegroup consisting of an acrylate group, a vinyl group, or combinationsthereof.

In the processes of the present invention, polymerizable monomers ormacromonomers may be attached to the surface of the silica-coatedpigment particles using any bifunctional compound having one groupcapable of bonding covalently to the silica coating and another groupcapable of covalently or ionically bonding to the monomers ormacromonomers. In one example, the compound may be a silane having atleast one polymerizable group, such as the polymerizable monomers listedabove (e.g. 3-(trimethoxysilyl)propyl methacrylate).

The polymeric stabilizer may be formed from a reactive and polymerizablemonomer or macromonomer which adsorbs, becomes incorporated or ischemically bonded to the bifunctional compound used to bridge the silicacoating and the polymeric stabilizer. The polymeric stabilizerdetermines the particle size and colloidal stability of the system andpreferably has a long polymeric chain which may stabilize the compositepigment particles in a hydrocarbon solvent.

In describing the reagents used to provide the desired polymerizable orinitiating functionality, we do not exclude the possibility that thepolymeric stabilizer may be “bifunctional.” For example, polymerizationinitiators are known (such as 4,4′-azobis(4-cyanovaleric acid)) havingmore than one ionic site, and such initiators may be used in the presentprocess. Also, as previously noted, a bifunctional compound may have theform of a macromonomer containing repeating units having the capacity tobond to the particle surface and other repeated units having the desiredpolymerizable or initiating functionality, and such macromonomericbifunctional compounds may form polymeric stabilizers that will normallycontain multiple repeating units of both these types.

The preferred class of functional groups for bonding to silica-coatedpigments are silane coupling groups, especially trialkoxy silanecoupling groups. One especially preferred reagent for attaching apolymerizable group to titania and similar pigments is theaforementioned 3-(trimethoxysilyl)propyl methacrylate, which isavailable commercially from Dow Chemical Company, Wilmington, Del. underthe trade name Z6030. The corresponding acrylate may also be used.

One type of macromonomer for use as a polymeric stabilizer may beacrylate terminated polysiloxane, such as Gelest, MCR-M11, MCR-M17, orMCR-M22, for example. Another type of macromonomers which is suitablefor the process is PE-PEO macromonomers, as shown below:

R_(m)O—[—CH₂CH₂O-]_(n)—CH₂-phenyl-CH═CH₂; or

R_(m)O—[—CH₂CH₂O—]_(n)—C(═O)—C(CH₃)═CH₂.

The substituent R may be a polyethylene chain, n is 1-60 and m is 1-500.The synthesis of these compounds may be found in Dongri Chao et al.,Polymer Journal, Vol. 23, no. 9, 1045 (1991) and Koichi Ito et al,Macromolecules, 1991, 24, 2348. A further type of suitable macromonomersis PE macromonomers, as shown below:

CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂.

The n, in this case, is 30-100. The synthesis of this type ofmacromonomers may be found in Seigou Kawaguchi et al, Designed Monomersand Polymers, 2000, 3, 263.

When choosing the bifunctional compound to provide polymerizable orinitiating functionality on the particle, attention should be paid tothe relative positions of the two groups within the reagent. As shouldbe apparent to those skilled in polymer manufacture, the rate ofreaction of a polymerizable or initiating group bonded to a particle mayvary greatly depending upon whether the group is held rigidly close tothe particle surface, or whether the group is spaced (on an atomicscale) from that surface and can thus extend into a reaction mediumsurrounding the particle, this being a much more favorable environmentfor chemical reaction of the group. In general, it is preferred thatthere be at least three atoms in the direct chain between the twofunctional groups; for example, the aforementioned3-(trimethoxysilyl)propyl methacrylate provides a chain of four carbonand one oxygen atoms between the silyl and ethylenically unsaturatedgroups, while the aforementioned 4-vinylaniline separates the aminogroup (or the diazonium group, in the actual reactive form) from thevinyl group by the full width of a benzene ring, equivalent to about thelength of a three-carbon chain.

In any of the processes described above, the quantities of the reagentsused (e.g., the organic core pigment particles, the silica shellmaterial and the material for forming the polymeric stabilizers) may beadjusted and controlled to achieve the desired organic content in theresulting composite pigment particles. Furthermore, the processes of thepresent invention may include more than one stage and/or more than onetype of polymerization.

As noted above, the particles made according to the various embodimentsof the present invention are dispersed in an encapsulation fluid. It isdesirable that the polymeic stabilizer be highly compatible with theencapsulated fluid. In practice, the suspending fluid in anelectrophoretic medium is normally hydrocarbon-based, although the fluidcan include a proportion of halocarbon, which is used to increase thedensity of the fluid and thus to decrease the difference between thedensity of the fluid and that of the particles. Accordingly, it isimportant that the polymeric stabilizer formed in the present processesbe highly compatible with the encapsulated fluid, and thus that thepolymeric stabilizer itself comprise a major proportion of hydrocarbonchains; except for groups provided for charging purposes, as discussedbelow, large numbers of strongly ionic groups are undesirable since theyrender the polymeric stabilizer less soluble in the hydrocarbonsuspending fluid and thus adversely affect the stability of the particledispersion. Also, as already discussed, at least when the medium inwhich the particles are to be used comprises an aliphatic hydrocarbonsuspending fluid (as is commonly the case), it is advantageous for thepolymeric stabilizer to have a branched or “comb” structure, with a mainchain and a plurality of side chains extending away from the main chain.Each of these side chains should have at least about four, andpreferably at least about six, carbon atoms. Substantially longer sidechains may be advantageous; for example, some of the preferred polymericstabilizers may have lauryl (C₁₂) side chains. The side chains maythemselves be branched; for example, each side chain could be a branchedalkyl group, such as a 2-ethylhexyl group. It is believed (although theinvention is in no way limited by this belief) that, because of the highaffinity of hydrocarbon chains for the hydrocarbon-based suspendingfluid, the branches of the polymeric stabilizers spread out from oneanother in a brush or tree-like structure through a large volume ofliquid, thus increasing the affinity of the particle for the suspendingfluid and the stability of the particle dispersion.

There are two basic approaches to forming such a comb polymer. The firstapproach uses monomers which inherently provide the necessary sidechains. Typically, such a monomer has a single polymerizable group atone end of a long chain (at least four, and preferably at least six,carbon atoms). Monomers of this type which have been found to give goodresults in the present processes include hexyl acrylate, 2-ethylhexylacrylate and lauryl methacrylate. Isobutyl methacrylate and2,2,3,4,4,4-hexafluorobutyl acrylate have also been used successfully.In some cases, it may be desirable to limit the number of side chainsformed in such processes, and this can be achieved by using a mixture ofmonomers (for example, a mixture of lauryl methacrylate and methylmethacrylate) to form a random copolymer in which only some of therepeating units bear long side chains. In the second approach, typifiedby an RGP-ATRP process, a first polymerization reaction is carried outusing a mixture of monomers, at least one of these monomers bearing aninitiating group, thus producing a first polymer containing suchinitiating groups. The product of this first polymerization reaction isthen subjected to a second polymerization, typically under differentconditions from the first polymerization, so as to cause the initiatinggroups within the polymer to cause polymerization of additional monomeron to the original polymer, thereby forming the desired side chains. Aswith the bifunctional reagents discussed above, we do not exclude thepossibility that some chemical modification of the initiating groups maybe effected between the two polymerizations. In such a process, the sidechains themselves do not need to be heavily branched and can be formedfrom a small monomer, for example methyl methacrylate.

Free radical polymerization of ethylenic or similar radicalpolymerizable groups attached to particles may be effected at elevatedreaction temperatures, preferably 60 to 70 C, using conventional freeradical initiators, such as azobis(isobutyryinitrile) (AIBN), while ATRPpolymerization can be effected using the conventional metal complexes,as described in Wang, J. S., et al., Macromolecules 1995, 23, 7901, andJ. Am. Chem. Soc. 1995, 117, 5614, and in Beers, K. et al.,Macromolecules 1999, 32, 5772-5776. See also U.S. Pat. Nos. 5,763,548;5,789,487; 5,807,937; 5,945,491; 4,986,015; 6,069,205; 6,071,980;6,111,022; 6,121,371; 6,124,411; 6,137,012; 6,153,705; 6,162,882;6,191,225; and 6,197,883. The entire disclosures of these papers andpatents are herein incorporated by reference. The presently preferredcatalyst for carrying out ATRP is cuprous chloride in the presence ofbipyridyl (Bpy).

RGP processes of the invention in which particles bearing polymerizablegroups are reacted with a monomer in the presence of an initiator willinevitably cause some formation of “free” polymer not attached to aparticle, as the monomer in the reaction mixture is polymerized. Theunattached polymer may be removed by repeated washings of the particleswith a solvent (typically a hydrocarbon) in which the unattached polymeris soluble, or (at least in the case of metal oxide or other denseparticles) by centrifuging off the treated particles from the reactionmixture (with or without the previous addition of a solvent or diluent),redispersing the particles in fresh solvent, and repeating these stepsuntil the proportion of unattached polymer has been reduced to anacceptable level. (The decline in the proportion of unattached polymercan be followed by thermogravimetric analysis of samples of thepolymer.) Empirically, it does not appear that the presence of a smallproportion of unattached polymer, of the order of 1 percent by weight,has any serious deleterious effect on the electrophoretic properties ofthe treated particles; indeed, in some cases, depending upon thechemical natures of the unattached polymer and the suspending fluid, itmay not be necessary to separate the particles having attached polymericstabilizers from the unattached polymer before using the particles in anelectrophoretic display.

As already indicated, it has been found that there is an optimum rangefor the amount of polymeric stabilizer which should be formed onelectrophoretic particles, and that forming an excessive amount ofpolymer on the particles can degrade their electrophoreticcharacteristics. The optimum range will vary with a number of factors,including the density and size of the particles being coated, the natureof the suspending medium in which the particles are intended to be used,and the nature of polymer formed on the particles, and for any specificparticle, polymer and suspending medium, the optimum range is bestdetermined empirically. However, by way of general guidance, it shouldbe noted that the denser the particle, the lower the optimum proportionof polymer by weight of the particle, and the more finely divided theparticle, the higher the optimum proportion of polymer. In general, theparticles should be coated with at least about 2, and desirably at leastabout 4, percent by weight of the particle. In most cases, the optimumproportion of polymer will range from about 4 to about 15 percent byweight of the particle, and typically is about 6 to about 15 percent byweight, and most desirably about 8 to about 12 percent by weight.

To incorporate functional groups for charge generation of the pigmentparticles, a co-monomer may be added to the polymerization reactionmedium. The co-monomer may either directly charge the composite pigmentparticles or have interaction with a charge control agent in the displayfluid to bring a desired charge polarity and charge density to thecomposite pigment particles. Suitable co-monomers may includevinylbenzylaminoethylamino-propyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid, 2-acrylamino-2-methylpropane sulfonic acid,2-(dimethylamino)ethyl methacrylate,N-[3-(dimethylamino)propyl]methacrylamide and the like. Suitableco-monomers may also include fluorinated acrylate or methacrylate suchas 2-perfluorobutylethyl acrylate, 2,2,2 trifluoroethyl methacrylate,2,2,3,3 tetrafluoropropyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropylacrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl acrylate, 2,2,3,3-tetrafluoropropylacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate or2,2,3,3,4,4,4-heptafluorobutyl methacrylate. Alternatively, charged orchargeable groups may be incorporated into the polymer via thebifunctional stabilizer used to provide polymerizable or initiatingfunctionality to the pigment.

Functional groups, such as acidic or basic groups, may be provided in a“blocked” form during polymerization, and may then be de-blocked afterformation of the polymer. For example, since ATRP cannot be initiated inthe presence of acid, if it is desired to provide acidic groups withinthe polymer, esters such as t-butyl acrylate or isobornyl methacrylatemay be used, and the residues of these monomers within the final polymerhydrolyzed to provide acrylic or methacrylic acid residues.

When it is desired to produce charged or chargeable groups on thepigment particles and also polymeric stabilizers separately attached tothe particles, it may be very convenient to treat the particles (afterthe silica coating) with a mixture of two reagents, one of which carriesthe charged or chargeable group (or a group which will eventually betreated to produce the desired charged or chargeable group), and theother of which carries the polymerizable or polymerization-initiatinggroup. Desirably, the two reagents have the same, or essentially thesame, functional group which reacts with the particle surface so that,if minor variations in reaction conditions occur, the relative rates atwhich the reagents react with the particles will change in a similarmanner, and the ratio between the number of charged or chargeable groupsand the number of polymerizable or polymerization-initiating groups willremain substantially constant. It will be appreciated that this ratiocan be varied and controlled by varying the relative molar amounts ofthe two (or more) reagents used in the mixture. Examples of reagentswhich provide chargeable sites but not polymerizable orpolymerization-initiating groups include 3-(trimethoxysilyl)propylamine,N-[3-(trimethoxysilyl)propyl]diethylenetriamine,N-[3-(trimethoxysilyl)propyl]ethylene and 1-[3-(trimethoxysilyl)propyl]urea; all these silane reagents may be purchased from UnitedChemical Technologies, Inc., Bristol, Pa., 19007. As already mentioned,an example of a reagent which provides polymerizable groups but notcharged or chargeable groups is 3-(trimethoxysilyl)propyl methacrylate.

In addition to the colored organic pigment particles, variousembodiments of the electrophoretic display media according to thepresent invention may further comprise at least one type of inorganicpigment particles. The inorganic pigment particles may also be coatedwith silica and a polymeric stabilizer, as described, for example, inU.S. Pat. No. 6,822,782. The white particles may be formed from aninorganic pigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄or the like. The black particles may be formed from CI pigment black 26or 28 or the like (e.g., manganese ferrite black spinel or copperchromite black spinel), carbon black, zinc sulfide, and combinationsthereof.

Generally, the four types of particles are divided into two groups—highcharge group and low charge group. In the two groups of oppositelycharged particles, one group carries a stronger charge than the othergroup. Therefore the four types of pigment particles may also bereferred to as high positive particles, high negative particles, lowpositive particles and low negative particles.

As an example, red particles (R) and white particles (W) may be thefirst group of oppositely charged particles, and in this group, the redparticles are the high positive particles and the white particles arethe high negative particles. The blue particles (B) and the greenparticles (G) may be the second group of oppositely charged particlesand in this group, the blue particles are the low positive particles andthe green particles are the low negative particles.

In another example, red particles may be the high positive particles;white particles may be the high negative particles; blue particles maybe the low positive particles and yellow particles may be the lownegative particles. It is understood that the scope of the inventionbroadly encompasses particles of any colors as long as the four types ofparticles have visually distinguishable colors.

As also shown in the FIGURE, a display layer utilizing the display fluidof the present invention has two surfaces, a first surface (17) on theviewing side and a second surface (18) on the opposite side of the firstsurface (17). The display fluid is sandwiched between the two surfaces.On the side of the first surface (17), there is a common electrode (15)which is a transparent electrode layer (e.g., ITO), spreading over theentire top of the display layer. On the side of the second surface (18),there is an electrode layer (16) which comprises a plurality of pixelelectrodes (16 a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in the FIGURE denotes apixel. As shown, each pixel has a corresponding pixel electrode. Anelectric field is created for a pixel by the potential differencebetween a voltage applied to the common electrode and a voltage appliedto the corresponding pixel electrode.

The percentages of the four types of particles in the fluid may vary.For example, one type of particles may take up 0.1% to 50%, preferably0.5% to 15%, by volume of the electrophoretic fluid.

It is also noted that the four types of particles may have differentparticle sizes. Useful sizes may range from 1 nm up to about 100 μm. Forexample, smaller particles may have a size which ranges from about 50 nmto about 800 nm. Larger particles may have a size which is about 2 toabout 50 times, and more preferably about 2 to about 10 times, the sizesof the smaller particles.

The density of the electrophoretic particle may be substantially matchedto that of the suspending (i.e., electrophoretic) fluid. As definedherein, a suspending fluid has a density that is “substantially matched”to the density of the particle if the difference in their respectivedensities is between about zero and about two grams/milliliter (“g/ml”).This difference is preferably between about zero and about 0.5 g/ml.

The solvent in which the four types of particles are dispersed is clearand colorless. It preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotri fluoride,chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Delaware, polydimethylsiloxane based silicone oil from Dow-corning(DC-200).

In one embodiment, the charge carried by the “low charge” particles maybe less than about 50%, or about 5% to about 30%, of the charge carriedby the “high charge” particles. In another embodiment, the “low charge”particles may be less than about 75%, or about 15% to about 55%, thecharge carried by the “high charge” particles. In a further embodiment,the comparison of the charge levels as indicated applies to two types ofparticles having the same charge polarity.

The charge intensity may be measured in terms of zeta potential. In oneembodiment, the zeta potential is determined by Colloidal DynamicsAcoustoSizer IIM with a CSPU-100 signal processing unit, ESA EN #Attnflow through cell (K:127). The instrument constants, such as density ofthe solvent used in the sample, dielectric constant of the solvent,speed of sound in the solvent, viscosity of the solvent, all of which atthe testing temperature (25° C.) are entered before testing. Pigmentsamples are dispersed in the solvent (which is usually a hydrocarbonfluid having less than 12 carbon atoms), and diluted to between 5-10% byweight. The sample also contains a charge control agent (Solsperse17000®, available from Lubrizol Corporation, a Berkshire Hathawaycompany; “Solsperse” is a Registered Trade Mark), with a weight ratio of1:10 of the charge control agent to the particles. The mass of thediluted sample is determined and the sample is then loaded into the flowthrough cell for determination of the zeta potential.

The magnitudes of the “high positive” particles and the “high negative”particles may be the same or different. Likewise, the magnitudes of the“low positive” particles and the “low negative” particles may be thesame or different.

It is also noted that in the same fluid, the two groups of high-lowcharge particles may have different levels of charge differentials. Forexample, in one group, the low positively charged particles may have acharge intensity which is 30% of the charge intensity of the highpositively charged particles and in another group, the low negativelycharged particles may have a charge intensity which is 50% of the chargeintensity of the high negatively charged particles.

Charge control agents may be used, with or without charged groups inpolymer coatings, to provide good electrophoretic mobility to theelectrophoretic particles. Stabilizers may be used to preventagglomeration of the electrophoretic particles, as well as prevent theelectrophoretic particles from irreversibly depositing onto the capsulewall. Either component can be constructed from materials across a widerange of molecular weights (low molecular weight, oligomeric, orpolymeric), and may be a single pure compound or a mixture. The chargecontrol agent used to modify and/or stabilize the particle surfacecharge is applied as generally known in the arts of liquid toners,electrophoretic displays, non-aqueous paint dispersions, and engine-oiladditives. In all of these arts, charging species may be added tonon-aqueous media in order to increase electrophoretic mobility orincrease electrostatic stabilization. The materials can improve stericstabilization as well. Different theories of charging are postulated,including selective ion adsorption, proton transfer, and contactelectrification.

An optional charge control agent or charge director may be used. Theseconstituents typically consist of low molecular weight surfactants,polymeric agents, or blends of one or more components and serve tostabilize or otherwise modify the sign and/or magnitude of the charge onthe electrophoretic particles. Additional pigment properties which maybe relevant are the particle size distribution, the chemicalcomposition, and the lightfastness.

Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, and is preferably soluble in the suspending fluid in an amountof at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyldecyne-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylenediamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 milligrams per gram (“mg/g”) of the particle mass, and morepreferably about 50 to about 200 mg/g.

In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Different non-limiting classes of charge control agents which are usefulinclude organic sulfates or sulfonates, metal soaps, block or combcopolymers, organic amides, organic zwitterions, and organic phosphatesand phosphonates. Useful organic sulfates and sulfonates include, butare not limited to, sodium bis(2-ethylhexyl) sulfosuccinate, calciumdodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basicbarium dinonylnaphthalene sulfonate, neutral or basic calciumdinonylnaphthalene sulfonate, dodecylbenzenesulfonic acid sodium salt,and ammonium lauryl sulfate. Useful metal soaps include, but are notlimited to, basic or neutral barium petronate, calcium petronate, Co-,Ca-, Cu-, Mn-, Ni-, Zn-, and Fe-salts of naphthenic acid, Ba-, Al-, Zn-,Cu-, Pb-, and Fe-salts of stearic acid, divalent and trivalent metalcarboxylates, such as aluminum tristearate, aluminum octanoate, lithiumheptanoate, iron stearate, iron distearate, barium stearate, chromiumstearate, magnesium octanoate, calcium stearate, iron naphthenate, zincnaphthenate, Mn- and Zn-heptanoate, and Ba-, Al-, Co-, Mn-, andZn-octanoate. Useful block or comb copolymers include, but are notlimited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides include, but are not limited to, polyisobutylenesuccinimides such as OLOA 371 or 1200 (available from Chevron OroniteCompany LLC, Houston, Tex.), or Solsperse 19000 and Solsperse 17000(available from Avecia Ltd., Blackley, Manchester, United Kingdom;“Solsperse” is a Registered Trade Mark), and N-vinylpyrrolidonepolymers. Useful organic zwitterions include, but are not limited to,lecithin. Useful organic phosphates and phosphonates include, but arenot limited to, the sodium salts of phosphated mono- and di-glycerideswith saturated and unsaturated acid substituents.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, non-aqueous surfactants may be used. These include, but arenot limited to, glycol ethers, acetylenic glycols, alkanolamides,sorbitol derivatives, alkyl amines, quaternary amines, imidazolines,dialkyl oxides, and sulfosuccinates.

If a bistable electrophoretic medium is desired, it may be desirable toinclude in the suspending fluid a polymer having a number averagemolecular weight in excess of about 20,000, this polymer beingessentially non-absorbing on the electrophoretic particles;poly(isobutylene) is a preferred polymer for this purpose. See U.S. Pat.No. 7,170,670, the entire disclosure of which is herein incorporated byreference.

Examples

The following examples are given as illustrative embodiments of thepresent invention, and are not intended to limit the scope of theinvention.

Sample 1

30 g of organic copper phthalocyanine blue pigments were dispersed in100 ml ethanol and 5 ml water solvent mixture. Then 1.5 g ammonia wasadded to raise the pH. 3 g of triethyoxysilane (TEOS) was added and themixture was stirred at room temperature for 20 hrs. After coating, theparticles were purified by a washing-centrifuging-redispersing processin ethanol three times.

The silica-coated organic pigment particles were then functionalized bymixing the silica-coated organic pigment particles with 30 gmethacryloxypropyl trimethoxysilane in 150 g methyl ethyl ketone (MEK)solvent and refluxed at 60-65° C. As-functionalized particles were thenpurified by a washing-centrifuging-redispersing process in isopropylalcohol (IPA) twice.

Polymer growth on the as-functionalized silica-coated pigments wascompleted by radical polymerization in toluene. 15 g of silanizedpigment was added into 50 g toluene and sonicated for 1 hour in a threeneck flask. 30 g of 2-ethylhexyl acrylate monomer was added into theflask and N₂ was purged into the flask for 20 minutes to remove oxygenand then the flask was heated to 65° C. 0.2 g of AIBN in toluenesolution was added into the flask. After 20 hours of reaction, theparticles were purified by a washing-centrifuging-redispersing processin toluene three times.

An electrophoretic ink media containing 24% polymer coated titaniumoxide particles, 16 wt % of blue particles from the example of Sample 1,5 wt % polymer coated red particles, 5 wt % of polymer coated yellowparticles and 0.4% Solsperse® 17000 and other charge adjuvant inisoparaffin solvent was prepared for optical-electric performancetesting.

Comparative Sample 1

The procedure used to produce Sample 1 was repeated except that copperphthalocyanine blue pigment was replaced with an inorganic cobaltaluminate blue spinel particle. Also, the encapsulated fluid containedmore than 15 wt. % of the inorganic blue pigment particles.

Comparative Sample 2

The procedure used to produce Sample 1 was repeated except that organiccopper phthalocyanine blue pigments were not coated with silica prior toforming the polymeric stabilizer.

Testing Method

Electrophoretic media was sealed between two transparent ITO-PETelectrodes through a microcell filling-sealing technique described inU.S. Pat. No. 6,859,302. The test sample was driven by a waveformgenerator using the same driving sequence. Measurement of the L*a*b*optical performance are conducted using X-rite iOne spectrophotometerunder a D65 illuminance setting.

The results from testing electrophoretic media containing the pigmentparticles according to Sample 1, Comparative Sample 1, and ComparativeSample 2, are provided below in Tables 1, 2, and 3.

TABLE 1 Electro-optical performance of electrophoretic media containingSample 1 Blue State Red State Yellow State L* 23.62 25.53 64.94 a* −4.0633.23 1.88 b* −22.01 17.52 44.76

TABLE 2 Electro-optical performance of electrophoretic media containingComparative Sample 1 Blue State Red State Yellow State L* 28 26.5 65.7a* −0.3 41.8 13.1 b* −14.2 28.5 60.8

TABLE 3 Electro-optical performance of electrophoretic media containingComparative Sample 2 Blue State Red State Yellow State L* 25 25.7 60.2a* −11.7 −7.4 −0.1 b* −22.8 −2.2 40.9

Comparing the results of the three different electrophoretic media, theelectro-optical performance of the colored particles of Sample 1provided unexpectedly improved results. Each of the optical states,blue, red, and yellow, for Sample 1 provided values indicating muchimproved color separation when compared to the particles of ComparativeSample 2, which demonstrated a degradation of the red state. Asexpected, the blue state provided by the organic particles of Sample 1provided superior electro-optical performance than the inorganicparticles of Comparative Sample 1. Thus, by providing the organicpigment particles with a hybrid silica and polymeric stabilizer coating,improved electro-optical performance was achieved.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

We claim:
 1. An electrophoretic display comprising: a front and a rearelectrode, at least one of the front and rear electrodes beingtransparent; and a dispersion fluid containing a plurality of pigmentspositioned between the front and rear electrode, the plurality ofpigments comprising a first, a second, and third type of organic pigmentparticle, and a fourth type of inorganic pigment particle; the firsttype of organic pigment particle having a first color and a first chargepolarity, the first type of organic pigment particle including a silicacoating and a polymeric stabilizer, wherein the polymeric stabilizer iscovalently bonded to a silane coupling group covalently bonded to thesilica coating, and wherein the first type of organic pigment particlescontains from about 4 to about 15 percent polymeric stabilizer by weightof the organic pigment particle; the second type of organic pigmentparticle having a second color and a second charge polarity, the secondcolor being different than the first color, and the second chargepolarity being the same as the first charge polarity; the third type oforganic pigment particle having a third color and a third chargepolarity, the third color being different than the first and secondcolor, and the third charge polarity being different than the first andsecond charge polarity; the fourth type of inorganic pigment particlehaving a fourth color and fourth charge polarity, the fourth color beingdifferent than the first, second, and third color, and the fourth chargepolarity being different than the first and second charge polarity. 2.The electrophoretic display of claim 1, wherein the dispersion fluidcontains a fifth type of pigment particle.
 3. The electrophoreticdisplay of claim 1, wherein the second type of organic pigment particlehas a surface treatment formed via dispersion polymerization.
 4. Theelectrophoretic display of claim 1, wherein the first, second and thirdtypes of organic pigment particle comprise an organic pigmentindependently selected from the group consisting of PB15:1, PB15:2,PB15:3, PB15:4, PR 254, PR122, PR149, PG36, PG58, PG7, PY138, PY150,PY151, PY154 and PY20.
 5. The electrophoretic display of claim 1,wherein the first, second, and third types of organic pigment particleare independently colored red, green, blue, cyan, magenta, or yellow. 6.The electrophoretic display of claim 1, wherein the fourth type ofinorganic pigment particle is black.
 7. The electrophoretic display ofclaim 6, wherein the fourth type of inorganic pigment particle isselected from the group consisting of metal oxides, manganese ferriteblack, copper chromite lack spinel, carbon black, and combinationsthereof.
 8. The electrophoretic display of claim 1, wherein the fourthtype of inorganic pigment particle is white.
 9. The electrophoreticdisplay of claim 8, wherein the fourth type of inorganic pigmentparticle is selected from the group consisting of TiO₂, ZrO₂, ZnO,Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄.
 10. The electrophoretic display of claim 1,wherein the polymeric stabilizer is derived from a monomer ormacromonomer.
 11. The electrophoretic display of claim 10, wherein themonomer is selected from the group consisting of styrene, alpha methylstyrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butylmethacrylate, t-butyl acrylate, t-butyl methacrylate, vinyl pyridine,n-vinyl pyrrolidone, 2-hydoxyethyl acrylate, 2-hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, lauryl acrylate, laurylmethacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexylacrylate, hexyl methacrylate, n-octyl acrylate, n-octyl methacrylate,n-octadecyl acrylate, n-octadecyl methacrylate, 2-perfluorobutylethylacrylate, 2,2,2 trifluoroethyl methacrylate, 2,2,3,3 tetrafluoropropylmethacrylate, 1,1,1,3,3,3-hexafluoroisopropyl acrylate,1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl acrylate, 2,2,3,3-tetrafluoropropylacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, and2,2,3,3,4,4,4-heptafluorobutyl methacrylate.
 12. The electrophoreticdisplay of claim 10, wherein the macromonomer contains a terminalfunctional group selected from the group consisting of an acrylategroup, a vinyl group, or combinations thereof.
 13. The electrophoreticdisplay of claim 10, wherein the macromonomer is an acrylate terminatedpolysiloxane.
 14. The electrophoretic display of claim 10, wherein themacromonomer comprises polyethylene oxide.
 15. The electrophoreticdisplay of claim 10, wherein the polymer stabilizer is formed usingrandom graft polymerization, ionic random graft polymerization, or atomtransfer radical polymerization.
 16. The electrophoretic display ofclaim 10, wherein the silica coating of the first type of organicpigment particle is 0.5 to 10 nm thick.
 17. The electrophoretic displayof claim 1, wherein the dispersion fluid is encapsulated.
 18. Theelectrophoretic display of claim 17, wherein the dispersion fluid isencapsulated within a microcapsule.
 19. The electrophoretic display ofclaim 17, wherein the dispersion fluid is encapsulated within amicrocell.
 20. The electrophoretic display of claim 17, wherein thedispersion fluid is encapsulated within a polymer matrix.